1. Field of the Invention
The invention relates to an exhaust emission control system favorably used in an internal combustion engine, and in particular to an exhaust emission control system of an internal combustion engine in which a NOx adsorbent is disposed in a bypass passage provided in an exhaust system of the engine.
2. Description of the Related Art
An exhaust emission control system of an internal combustion engine in which a bypass passage is provided in an exhaust system and a NOx adsorbent is disposed in the bypass passage is known, as disclosed in, for example, Japanese Patent Application Publication No. 2006-342700 (JP-A-2006-342700).
Upon cold start of the engine, for example, the above type of exhaust emission control system causes exhaust gas to flow through the bypass passage. Thus, when the engine is started while a catalyst has not been warmed up, the system allows the NOx adsorbent to adsorb a NOx component contained in the exhaust gas, thereby to improve the quality of exhaust emissions during starting of the engine.
In the exhaust emission control system as described above, the NOx component in the exhaust gas is adsorbed by the NOx adsorbent. However, the NOx adsorbing capability of the NOx adsorbent may drop off or degrade depending on, for example, an environment in  which the adsorbent is used. The degradation in the adsorbing capability may be a surmountable degradation which is caused by reversible changes in the structure of the adsorbent and from which the adsorbent can recover, or an insurmountable degradation (permanent deterioration) which is caused by irreversible structural changes in the adsorbent and from which the adsorbent cannot recover.
In the exhaust emission control system as described above, the above two types of degradation in the adsorbing capability are not discriminated from each other. In the above-described emission control system, therefore, wasteful regeneration control, or the like, may be performed so as to resume the adsorbing capability, even in the case where permanent deterioration occurs in the NOx adsorbent. In this case, the NOx adsorbent continues to be used without being recovered from the degradation of the adsorbing capability, resulting in deterioration of exhaust emissions during starting of the engine. In addition, if fuel injection and other engine operations are performed under useless regeneration control, the exhaust emissions may be further deteriorated.